Many biologic, especially electrical, parameters are measured by the difference in electrical potential across specific organs or organ systems. Specifically, electrocardiograms (ECG) and other electrical parameters of cardiac activity require two or more electrodes or sensing devices placed so that they are on each side of the electrical axis of the heart. The basic arrangement, known as Lead I, consists of strapping an electrode to each wrist so as to measure the electrical potential difference across the heart. While such an arrangement is perfectly acceptable for clinical testing, it is inconvenient and impractical, if not impossible, in other circumstances such as during routine exercise programs.
A growing number of individuals are performing exercise programs regularly or intermittently. It is generally acknowledged that the heart rate is the best indicator of the value of such exercise to the cardiovascular system, i.e., when enough effort is expended to be useful and when so much effort is expended as to be dangerous. Thus, as reported in L. R. Zohman, Exercise Your Weight to Fitness and Heart Health, 16-18 (CPC International, 1974), it is important in an effective exercise training program that there be a gradual increase in heart rate until a target rate of 70 to 85 percent of the maximum attainable rate (depending on age and physical condition) is achieved. A period of 20 to 30 minutes of exercise at the target level will then provide a significant conditioning effect on the cardiovascular system. While the average target zones for different age groups are readily available (for an average 40-year old male it is, for example, 128 to 155 beats per minute), it is apparent that most individuals lack the skill or inclination to take their own pulse at various intervals during the exercise period to ascertain whether their heartbeat rate falls within the target range.
The difficulties and inconveniences attending the attachment of bipolar electrodes to the wrists or to other spaced points on opposite sides of the heart and the obvious problems in exercising with such electrodes in place have lead others to pursue the development of mechanical devices for sensing the pulse and then converting the mechanically-sensed pulse to an electrical signal for processing and display. U.S. Pat. No. 3,807,388, for example, discloses a heartbeat rate monitor in the form of a wrist-watch having a transducer mounted in the wrist strap to detect the pressure change (pulses) and convert them to electrical signals.
Apart from the fact that differences sometimes exist between pulse rate and true heartbeat rate, substantial problems exist in providing a transducer which is easily positioned for proper operation and which is capable of accurately sensing the pulse for any of a variety of users. Therefore, despite the availability of technology and electrical hardware for processing and displaying in digital form the pulse rate of a patient once that pulse has been detected by a suitable pressure transducer, devices in wristwatch form or in other convenient portable form have not achieved widespread use or substantial commercial availability.
Other patents illustrative of the art are U.S. Pat. Nos. 3,863,626, 3,802,698, 3,948,250, 3,586,835, and 3,717,140. Reference may also be had to V. Elings et al, A Cardiotachometer which Calculates Rate Digitally, IEEE Transactions on Biomedical Engineering, 468-470 (November, 1973), and Hartley, Analogue-Display Rate Meter Build Around Digital Switching Elements, Medical and Biological Engineering, 107-108 (January, 1976).